1. Technical Field
The present invention relates to a projector and more particularly to a projector capable of displaying both visible image and infrared image on a screen.
2. Related Art
A projector which displays an enlarged optical image on a screen as image information created by using an optical modulation element has high adjustability to digital equipment such as a computer. Thus, this type of projector has been widely used for the purpose of presentation, information, showing pictures, and other purposes. Since the chief function of a general display apparatus including a projector is to provide some implicational information for a person, image information to be displayed is generally given as a visible image which is visible to human eyes. Accordingly, a typical projector selects visible light from light emitted from a light source, and displays an image by using the selected visible light as illumination light.
For displaying implicational information for the purpose of presentation or information, it is considered that the volume of information appropriate for display is limited from the viewpoints of visibility, readability, receivability of information, and other points. When excessive volume of information is on display at a time, a person cannot receive information with sufficient accuracy just as a case where he or she has difficulty in obtaining necessary information in a busy shopping area of a city crowded with signboards. In this case, there is even a tendency that he or she pays no attention to respective information.
In case of presentation or picture showing, security is also required for an image to be displayed. Shooting and using presentation information or picture information for other purposes without permission of the person having copyright of the information has been currently a serious social problem.
For overcoming these problems, a technology for controlling display information (image and picture) by using infrared light invisible to human eyes has be under development. An image displayed by infrared light (infrared image) cannot be visible to humans, but the display contents of the image can be recognized by using an image shooting element which has sensitivity throughout the infrared range such as a CCD and a CMOS sensor. According to a technology disclosed in Proposal of information Presentation Method in Actual Environment Using IR Projector, information Processing Technology Letters (FIT2003), Vol. 2, pp. 287-289, 2003, written by Yoshinari Shirai and Mitsunori Matsushita, (see FIG. 1), for example, a method for controlling information (image and picture) to be displayed by using a combination of an ordinary visual light projector which displays visual color images and an infrared light projector which displays images by infrared light is proposed. More specifically, information to be provided for an unspecified number of persons such as information about the presence of the information and the outline of the information is displayed as a visible image, and information to be provided only for a person requiring more detailed information is displayed as an infrared image. The invisible infrared image is observed via an image shooting element such as a digital camera.
In view of security for a display image, U.S. Pat. No. 6,018,374 proposes a method for preventing sneak shot or the like by providing notification that shooting without permission of the person having the copyright is an illegal act, for example. According to this method, an infrared projector overlaps and projects an infrared image on an image projected on a screen. When an image shooting element such as digital video and digital camera shoots the image projected on the screen, the shot image contains the infrared image.
As described in the above references, the method of overlapping and displaying the invisible infrared image on the visible image shown on the screen is effective for controlling display image (image and picture) and preventing illegal acts such as sneak shot. In this method, however, an additional second projector for projecting an infrared image is required as well as a projector for projecting and displaying original information. Considering that the number of portable projector is increasing as a result of size reduction of projector, addition of the second projector lowers portability. Moreover, the display positions of the two projectors during display need to be accurately matched. In this case, the projection display positions of the two projectors need to be accurately matched every time the projectors are moved. However, particularly for the infrared projector which projects invisible infrared images, the display position cannot be visually controlled. Accordingly, freedom of installation and easiness of use extremely lower.